FIELD OF THE INVENTION
The present invention is in the field of synthesis methods for the preparation of commercial cephalosporin antibiotics, of which there are presently a significant number, these therapeutic agents now being in their fourth generation. The large variety of side chains to be found in commercial cephalosporins and the significant economic importance of the cephalosporins has placed increased importance on achieving more economic and efficient methods of preparing key intermediates which permit ready synthesis of the various cephalosporins.
One of these key intermediates is 7-amino-cephalosporanic acid (7-ACA), which may be represented by the following formula: ##STR1## Currently, 7-ACA is produced from Cephalosporin C. Cephalosporin C itself is a fermentation product which is the starting point for nearly all currently marketed cephalosporins. However, synthetic manipulation to produce these various commercial cephalosporins basically starts in most cases with the 7-aminocephalosporanic acid, which must be derived from the Cephalosporin C by cleavage of the 7-aminoadipoyl side chain. Typical commercial cephalosporins derived synthetically from 7-ACA, and which thus have the 3-acetyloxymethylene side chain, include cefotaxime, cephaloglycin, cephalothin, and cephapirin.
Another of the key intermediates is 7-aminodeacetylcephalosporanic acid (7-ADAC), which may be represented by the following formula: ##STR2## Currently, 7-ADAC is also produced from Cephalosporin C by removal of the 7-D-.alpha.-aminoadipoyl side chain, together with conversion of the 3-acetyloxymethyleneside chain to 3-hydroxymethyl. 7-ADAC is a useful intermediate compound in the synthesis of cephalosporins containing modified substituents at the C-3 position.
Currently, the method of choice in the art for cleaving the 7-aminoadipoyl side chain is chemical. The basic imino-halide process requires blocking of the amino and carboxyl groups on the 7-aminoadipoyl side chain, and several methods for accomplishing this are currently used. However, as presently employed, the chemical cleavage process has serious disadvantages. Among these are the requirements of a multi-step and complex process, extremely low operating temperatures, expensive reagents, significant quantities of process by-products resulting in effluent treatment problems, and purification of a highly impure starting material before chemical treatment begins. Consequently, there has been an ongoing search for a microbiological or fermentative process which would achieve enzymatic deacylation of Cephalosporin C to provide 7-aminocephalosporanic acid on a more economic basis than the chemical process currently in use.
However, this search for a successful microbiological process has largely proved futile. This is a result, as is made clear in the literature, of the structure, and especially the stereochemistry, of the aminoadipoyl side chain of the Cephalosporin C molecule, since penicillin has been successfully deacylated by enzymatic cleavage using penicillin acylase produced by a variety of microorganisms. Reports of successful one-step enzymatic deacylation of Cephalosporin C in the literature, on the otherhand, are often unreproducible or provide only very marginal yields.
Accordingly, the present invention is particularly in the field of preparing the key cephalosporin intermediate 7-ACA, and more particularly, in the field of bioprocesses for the preparation of 7-ACA.
To date, the search for a successful bioprocess for making 7-ACA has largely proved futile, certainly with respect to one of commercial scale. For example, while it has been possible to prepare 6-amino penicillanic acid (6-APA) by direct fermentation and/or by enzymatic treatment of penicillin G, leaving only ring expansion necessary to give 7-ADCA, it has been found that, unfortunately, the Cephalosporium or Streptomyces enzymes which carry out ring expansion in the normal metabolic pathways of these microorganisms do not accept 6-APA as a substrate. These enzymes, which are collectively referred to in the art as the DAOCS or expandase enzyme, are defined as enzymes which catalyze the expansion of penam ring structures found in penicillin-type molecules to ceph-3-em rings, as found in the cephalosporins. Hereafter, these enzymes will be referred to collectively as "the expandase enzyme".
A substrate on which the expandase enzyme does operate is penicillin N, which upon ring expansion and hydroxylation, gives deacetylcephalosporanic acid (DAC). Here, it is only necessary to cleave the (D)-.alpha.-aminoadipoyl side chain to give 7-ADAC, but this side chain has proven stubbornly resistant to enzymatic cleavage, giving only unacceptably low yields.
In accordance with the present invention it has been possible to achieve an efficient bioprocess wherein a penicillin compound (having an adipoyl side chain) is produced by a novel fermentation process in high titers, said penicillin compound being an acceptable substrate for the expandase enzyme which is produced in situ by the same microorganism which produces the penicillin compound, having been transformed to express said expandase enzyme. The expandase enzyme then operates to ring expand the penicillin compound to a cephalosporin compound in high yields.
The adipoyl-7-ADCA produced by in situ action of the expandase enzyme has a 3-methyl (--CH.sub.3) side chain, whereas 7-ACA, the final product, has a 3-acetyloxymethyl [--CH.sub.2 OC(O)CH.sub.3 ] side chain. In order to convert the 3-methyl to a 3-acetyloxymethyl side chain, in accordance with the present invention there is also expressed in situ two further enzyme activities in addition to the expandase activity. These are, in order, an hydroxylase and an acetyltransferase, and both are the expression products of genes with which the microorganism producing the penicillin compound has also been transformed. The hydroxylase enzyme converts the 3-methyl side chain of adipoyl-7-ADCA to 3-hydroxymethyl, and the acetyltransferase enzyme converts this 3-hydroxymethyl side chain to the 3-acetyloxymethyl side chain of 7-ACA.
And, importantly in the last critical step of the method of the present invention, the side chain of the penicillin compound, now a cephalosporin compound, is removable by another enzyme system in surprisingly high yields. The unexpected result of this unique, total bioprocess which comprises the present invention, is the production of 7-ACA in surprisingly high yields, and with sufficient economy to represent a reasonable alternative to currently used methods of chemical and biochemical processing.